The invention relates to a process for the production of finished paper having a greatly improved wet and dry strength, sizing and intrinsic retention using a) polyisocyanates having anionic or potentially anionic groups and b) polymers having cationic or potentially cationic groups.
EP-A 582 166 relates to water-dispersible polyisocyanates having tertiary amino groups and/or ammonium groups, i.e. having cationic or latently cationic groups, and optionally having polyether groups, and to a process for the production of cellulose-containing material, which has been given a dry strength and wet strength treatment and/or sized, using these water-dispersible polyisocyanates. The co-use of cationic auxiliaries, such as, for example, retention agents, is referred to, retention agents which are mentioned being customary cationic retention agents and so-called dual systems comprising cationic or cationic or anionic and, if appropriate, individual components, such as silica sols. Additional retention agents are not co-used in the examples of EP-A 582 166. The reader thus gains the impression that retention agents should at the most be used together with cationic polyisocyanates if they comprise at least partly, but preferably entirely, cationic products, but a reason for this is not given; the advantages or disadvantages are not discussed.
German Offenlegungsschrift 4 211 480 relates to a process for increasing the wet strength of paper using water-dispersible polyisocyanates. If appropriate, the products can also be employed with customary cationic fixing agents and retention agents, preferably together with agents for increasing the wet strength. Retention agents which are mentioned in this context are cationic polycondensates of polyamines, preferably N-methyl-bis-(3-aminopropyl)-amine, and alkylene dihalides, preferably dichloroethane. The effects of this joint use of water-dispersible polyisocyanate and cationic retention agent are described in Example 4 of German Offenlegungsschrift 4 211 480. The data of Example 4 show that the joint addition 1. of a polyisocyanate containing polyethylene oxide groups and 2. of a cationic retention agent indeed leads to a marked (desired) increase in the ash content, but also leads to an (undesirable) reduction in the wet strength. Although according to German Offenlegungsschrift 4 211 480 polyisocyanates containing carboxyl groups are mentioned as preferred water-dispersible polyisocyanates which can easily be dispersed in water after at least partial neutralization of the carboxyl groups, only non-ionic water-dispersible polyisocyanates (containing polyether groups) are employed in the examples.
Surprisingly, it has now been found that not only the retention, which is reflected in the ash content (in the case of paper grades containing filler), but also the dry and wet strength and the sizing can be improved if water-dispersible polyisocyanates having anionic (or potentially anionic) groups and, as retention agents, cationic (or potentially cationic) compounds are employed.